As the concentration of carbon dioxide (CO2) in the atmosphere keeps increasing, serious concerns have been raised with respect to its impact on the environment. Since it started being monitored in 1958, the increase of CO2 concentration in the atmosphere has accelerated from less than 1 ppm/year prior to 1970 to more than 2 ppm/year in recent years. As a result, the atmospheric level of CO2 increased from 315 ppm in 1958 to 385 ppm in 2009. CO2 is considered to be the main anthropogenic contributor to the greenhouse gas effect, as it is allegedly responsible for 60% of the increase in atmospheric temperature, commonly referred to as “global warming”. Among the various sources of CO2, approximately 30% is generated by fossil fuel power plants, and 25% from transportation sector, making them major contributors to global warming. Despite their impact on the environment, it is acknowledged that fossil fuels will remain the leading source of energy for years to come in the word, for both power generation and vehicle transportation.
Although CO2 emission from the transportation sector represents only one third of the overall CO2 emissions in the world, it denotes the highest source of emissions in urban populated areas. This situation is undoubtedly critical for the quality of air in populated regions and may cause severe health and societal problems. CO2 capture and storage in vehicles is certainly a challenging problem, but due to its small scale, it is still technically more difficult than CO2 capture in a stationary power plant.
With regards to CO2 separation, liquid amine scrubbing is currently practiced on a large scale for the purification of industrial gases (e.g., natural gas, syngas) and also in life support systems in confined space (e.g., submarines, space shuttle). Amine scrubbing processes use alkanolamine aqueous solutions, the most common being mono- and di-ethanolamines, (MEA and DEA) and N-methyldiethanolamine (MDEA).
The use of aqueous solutions of low molecular weight alkanolamines suffers a number of drawbacks. Under scrubbing conditions, (i) a fraction of the amine and its decomposition products are lost by evaporation, which in addition to reducing the absorption capacity, may cause problems because of their toxicity, (ii) the amine undergoes oxidative degradation leading to decreased capacity, increased viscosity and excessive foaming, (iii) excessive corrosion takes place, thus posing severe operational problems. Moreover, for small scale applications, the current technology for wet acid gas removal remains economically challenged. Therefore, there is a persistent need to develop more efficient and economical processes and materials.
In addition to liquid phase systems, there were attempts to use solid or amine (impregnated and grafted), supported materials (mainly silica), particularly for air revitalization in manned space shuttles and CO2 capture. The most common non-reactive acid gases adsorbent, i.e., zeolite 13X, is highly efficient for removing low level CO2, but it requires high temperature activation (>350° C.), and its cyclic adsorption performance in the presence of moisture decreases dramatically, which requires a dehydration unit and acid gases removal prior to the CO2 adsorption bed.